Switching network control system



April 14, 1964 M. c. PAULL 3,129,407

swncumc NETWORK CONTROL SYSTEM Filed Nov. 24, 1961 15 Sheets-Sheet 1INPUT 872165 I OUTPUT 671465 0 [L0 1L1: '"Qijggg 0L0 am 0/ a o/ n 4/ n aa I Ii I: I:

I I I ill 1: SI H101} I I I z l l I I n i i t Iosol @3 I 15/ OS/ HORIZON7:41. 647' E C/RCU/ T HORIZON T41. GA 7' E C/RCU/ T CONTROL CIRCUITNETWORK CONTROL uvpur u/ve OUTPUT um; I IDENTITY mewr/ry VEN TOR M. C.PA ULL ATTORNEY April 14, 1964 Filed Nov. 24, 1961 M. C. PAU LL 15Sheets-Sheet 2 F IG. 2 (NETWORK CONTROL 100) A35; ourpur u/w: IDENTITYH32 02 OUTPUT TCHI OUTPUT I REG. LEVEL J NETWORK *1 l L 1 REG. 1 III LACCESS CIRCUIT cou/vr 1 PEG. 1

REG. 1 L I607 1 INTER/M LOG/c cou/vrm rmsm' I 1 10/ /80\C0MPARISON r 1*ma CIRCUIT cou/vr t L REG.- ACCESS CIRCUIT 124 me; f INPUT INPUT REG.3%,}; zvzrwomr f MEMO lour ur LEVEL 1 r f REG. J

T /45 INPUT u/vs IDENTITY t Hfl J Ll|--|---+ II I l REQUEST 7 SERVEDApril 14, 1964 Filed Nov. 24, 1961 SWITCHING 15 Sheets-Sheet 3 F IG. 3(NETWORK CONTROL I00) ACCESS /3/ c/RGu/r INPUT l r/30 our ur R k, CHANGE--r ORV SW/TCH OUTPUT LEVEL ,ourpur SWITCH ,INTERMED/ATE SWITCH To SWNE]: [INPUT SWITCH CONTROL /2 UP/DOW/v J /'l OUTpUT r/55 INPUT m REG./50 15a /54 T f f f t oRoER ACCESS oRoER MEMORY C/RCU/T COUNTER f l tAGcEss c/Rcu/T v UP/DOW/V IN. snr l/VTERM. sn: LEVEL ourpur our SmSWITCH REG. INPUT fr CHANGE -t INPUT I MEMORY REG. 1 /42 /4/ ACCESSCIRCUIT REG.

April 14,- 1964 M. c. PAULL SWITCHING NETWORK CONTROL SYSTEM 15Sheets-Sheet 4 F/G.4B

Filed Nov. 24, 1961 FIG 4A (COMPOSITE NETWORK) a mmma 2 J 2 3 2 3 LEVEL03 COLUMN OUTPUT SWITCH NUMBERS C2 C5 Cl C3 C4 (OUTPUT NETWORK MEMOR no)8 m R w A TQHL T2, 0 u L m 2 P w, p w w wE I USL USL W O 0 T U 3 w w w ww I WWJE/ WW-EZ O Q w B n Q q 4 A 0 n C 4 M u a F 0 a 03 A an a 2 A M M/n d 3 2 a April 14, 1964 M. c. PAULL 3,129,407

SWITCHING NETWORK CONTROL SYSTEM Filed. Nov. 24. 1961 15 Sheets-Sheet 5F IG. 6 FIG. 8 (INPUT NETWORK MEMORY /2o) (OUTPUT CHANGE MEMORY lao)ourpur SWITCH ADDRESS 0/ 02 0a SWITCH LEVEL INPUT .9/ sw. 1/, o2

LEVEL 2 INPUT 2 2 s2 sw. I2,

LEVEL 2 l INPUT INPUT 52 SW. 12, sw. II,

LEVEL .2 LEVEL .3

F IG. 7 FIG. 9 (ORDER MEMORY 15a) (INPUT CHANGE MEMORY 14o) SWITCH LEVELnow/v 1/, a sa 7 oa,2

3 UP II, 83 02,2

U7 INPUT INT E RM. OUTPUT DOWN SWITCH SWITCH SWITCH April 14, 1964 M. c.PAULL 3,129,407

SWITCHING NETWORK CONTROL SYSTEM Filed Nov. 24, 1961 2 15 SheetsSheet 6FIG. I0 (LOG/C I60) FIG. /3A FIG. I38 I63 (02 GATE) E :9" A 16/ 8 our T0.co/vmoz.

154 E FIG. /4A FIG. I48 I62 A FIG. I/ OUT (COMPARISON ccT. laa) I FIG.I5

FIG. FIG.

m /7 i FROM FROM '7 2 255.113 7 c./as

I 1 P26! 1 FROM FIG. FIG. REG/23 I 1 250.143 20 2/ I I L 1 F/G.

FIG. IZA FIG. [28 Ha.

(A/vo GATE) 2.; :D A

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April 14, 1964 M. c. PAULL 3,129,407

SWITCHING NETWORK CONTROL SYSTEM Filed Nov. 2.4, 1961 15 Sheets-Sheet 7FIG. /6

comvcr/o- FROM REOUES TED smrs 10 STATE 40.

our ur REG. /33- ACCE$$ l2! ourpur REG. /43-ACCE$S u/ COUNTER IOI+ACCE$$n2 COUNTER IOI+ACCESS I22 READ OUT NETWORK cou/v TER /o/+ coum' REG. I02cou/v TER lol cou/vr REG. ma

April '14,. 1964 M. c. PAULL 3, 7

SWITCHING NETWORK CONTROL SYSTEM Filed 1961 FROM 8mm 15 Sheets-Sheet 8FIG. I7

FROM sum 58 STAA'E 1- N0 REQO/V LEAD /7/ REo.o/v LEAD 0P STATE 2 COUNTER/o/- AccEss lam/42 LEAD /35- /NPUT REG. m LEAD /45 INPUT REG. /4/

WRIE IN CHANGE MEM. I30, I40

STAIE 3 COUNTER /o/ ACCESS /a2,/42

READ OUT CHANGE MEM. I30, I40

STAIE 4 RECORD RE CORD RELEASE CONNE C T ION DISCONNECT/ON RE QUES TE 0STATE 4 b STATE 4 C WRITE IN NE T.' MEM. //0,/20 WRITE IN NET. MEM.//0,/20

ADV. COUNTER I54 STATE 8 COUNTER /0/-+cou1vr REG. /0.:

uP'-' srArE REG. /05

READ OUT COUNTER /0/ April 14, 1964 MC. PAULL 3,129,407

SWITCHING NETWORK CONTROL SYSTEM Filed Nov. 24, 1961 5 Sheets-Sheet 9READ OUT COUNTER I0! A I FIG. RE DOUT COUNTER O STA QE 9 cow/r51? 1011vor=n COUNTER 101:

)5 T COUNTER /0/ TO I FROM STATE IO r- STATE 10 STATE l4 ADVANCE co r590 COUNT REG. /02ACCE$S [/2 OUTPUT REG. l43-ACCESS COUNT REG. IDS-ACCESSI22 OUTPUT REG. [JG-*ACCESS l2! READ our -51: HEM. 110, 120

STATE IS COUNTER /0/- SA| E REG. I04 COUNTER lOl-SAVE REG. I04

STATE IS ourPur REGJ/J-HNPUT REG. 1a1

ourpur REG. 12.2-11v1=ur R56. 14/ READ cowvrsn 1o1-.4ccss 1.22, 142 READour 1 COUNTER 101 I WRITE/N CHANGE COUNTER 0/ MEM. 130,140

TO STATE 4 READ our cmwas MEM. 130,140

STATE l8 April 14, 1964 M. c. PAULL 3,129,407

SWITCHING NETWORK CONTROL SYSTEM Filed Nov. 2 1961 STAT g ll SAVE REC.I04 541/5 REG. I04

NOT: I

N 3/0. on LEAD m 401 COUNTER I54 70 8714TE/ 357' ALL c og/vrms STAKE I2STARE 59 COUNTER IS4- ACCES$ m LEAD m REO. LEAD/7/ R50. ALL ORDERS-ACCE8S /52 0/sc0mv5cr 0/sc0/wv5cr (INPUT sw. c/v/v) (OUTPUT sw. GIVENREAD our 0R05R MEM. I

TO STATE 58 v STAKE 6O OUTPUT REG. m ACCE$$ 12/ cou/vr5R /o/- ACCESS//z,/22

READ our -57: MEM. 120

STATE 6| STATE 63 J OUTPUT c. /2.3 LO6"/C I OUTPUT REG. /2.: ACCE$$ 11/c= c='0' COUNTER /0/ ACCE$$ //z R5A0 0ur/- /v r M M. //o

, sTgE 62 A 56%655233'3 =7! OUTPUT REG. 1mm COMP/IR. /ao

COMP. /ao COMP. /ao="o* ADVANCE To STATE 58 COUNTER /o/ STA';\E OUTPUTREG. /2J /NPUT R5a./4/ COUNTER /0/ ACCESS /42 wR/r5 //v CHANGE MEM. /40

T0 mm as April 14, 1964 M. c.' PA'ULL SWITCHING NETWORK CONTROL SYSTEMFiled NOV. 24, 1961 FIG. 20 1 FROM STA TE 30 STATE I 9 cou1vr REG. 102INPUT REG. 151

INTERM. $W.- ACCES$ 152 WRITE //v o oER MEM. 150

STATE 2| OUTPUT REG. 1aa /1v/=ur REG. 15/

OUTPUT $W. ACCES$ 152 WRITE IN ORDER ME M. I50

STATE 23 A STATE REG. /05 /NPUT REG. 151

/pow-- ACCESS /52 COUNTER cou/vTER 1o1= No 7': SA V5 REG. 104

COUNTER 10/ DECREMENT COUNTER I0/ 8 WRITE IN ORDER MEM./50

V STATE 25 COUNTER /0/ ACCE$$ 142 j READ OUT CHANGE ME M. I40

STATE 2 7 WRITE IN ORDER MEM. I50

STATE 2 9 ADVANCE ORDER COUNTER I54 70 STATE 20 Sheets-Sheet 11 FROM 6TA TE 29 STATE A cowvr REG. 1o: -11vPur REG. 15/

INTERM. SM ACCE$S 152 WRITE IN ORDER MEM. I50

STATE 22 ourpur REG. /43 /NPU7' REG. 15/

INPUT SW.ACCESS 152 WRITE IN ORDER MEM. I50

STATE 24 $12155 REG. /o5 /1v1=ur REG. 15/ UP/D0W- ACCE$$ 152 COUNTER101: 7" COUNTER 10/ SA V5 REG. 104 NO7=/ COUNTER 10/ DECREMENT READ OUTCHANGE MEM. I

STATE 28 ourpur REG. /aa-1-PuT REG. 15/

ourpur sw. AGcEsG 152 WRITE IN ORDER MEM. I

STATE 3 O ADVANCE ORDER COUNTER I54 TO STATE I9 pril 14, 1 64 M. c.PAULL 3,1

swncnme NETWORK CONTROL SYSTEM Flled Nov. 24, 1961 15 Sheets-Sheet 12STATE 68 A F OUTPUT REG. /4a- AccE$s m COUNTER l0/ ACCE$$ 112,22

READ OUT NE T MEM. l/O

STATE 7| STATE 70 A A OUTPUT REG. //3 ACCE$$ /2/ COUNTER /0/=n COUNTERNor-m COUNTER /o/- ACCESS //2,/22

READ OUT NET.

ME'M. I20

ADVANCE STATE 72 To sure 58 COUNTER /o/ OUTPUT REG. /za,/4a cam /80COMP=/ COMP=0 STATE 73 OUTPUT REG.//3 /NPUT REG./3/ COUNTER /0/ ACCE$S132 J WRITE //v CHANGE ME'M. uo FROM .5714 T5 as STATE 67 WRITE IN ORDERMEM. /50

TO STATE 36 April 14, 1964 M. c. PAULL 3,129,407

SWITCHING NETWORK CONTROL SYSTEM Flled Nov, 24, 1961 15 Sheets-Sheet 13STAT IE. 3| cOu/vrER ACCES$ /:2./42

READ our CHANGE MEM. /J0,/40

STA';\E 32 STATE REG. STATE REG. /0:

READS DOWN READS UP FROM FROM STATE 44 am TE 4:

STAT/1E 33 STATE 34 COUNT REG. l03 /NPUT REG. /5/ STAJ'E REG. /05 /NPU7'REG. /5/

INTER/4. $W.- ACCE$$ I52 UP /DOWN ("7466555 /52 wR/rE //v ORDER WRITE//v ORDER MEM. o MEM. 15o

STAKE 35 STATE 36 OurRur REc. ma -/NPUT REG. /5/ COUNT REG. /02 /NPUTREG. /5/ OUTPUT sw. +4 ccEss /52 INTERM. SW. ACCES.S /52 UP $TA rE REG.I05

WRITE //v ORDER WRITE IN ORDER MEM. I MEM. I50

STARE 37 STAT/\E 38 $7I1TE REG. /05- /NPU7' REG. /5/ /0UTPU7' REG. /4a//vRur REc./5/ UP/ c- I52 INPUT $PV.- ACCE$$ I52 cOu/vrER /o/ cOu/vrER/o/ COUNTER /o/ COUNTER /o/ 1/ Ill 1/ I NOT l/ DECREMENT DECREMENT WRITE//v COUNTER /o/& COUNTER 10/ ORDER MEM. 15o WRITE m ORDER MEM.

STATE 39 STATE 40 A A COUNTER /0/ ACCES5 /:2./42 COUNTER /O/ AccEss/a2,/42

April 14, 1964 c PAULL 3,129,407

SWITCHING NETWORK CONTROL SYSTEM Filed NOV. 24, 1961 l Sheets-Sheet 14 F/G. 23

STA'IE 4| sTAT E 42 ourpur R56. R1: //VPU7' REG. 1:? /0UTPUT R56. I33/NPU7' REG. INPUT SW. ACCE$$ /sz OUTPUT $W. ACCE$$ we wR/rE //v oRoERMEM. 150

STATE 43 STATE 44 A J;\

I REQUEST TO STATE TO .STATE RELEASE ADVANCE ORDER 34 J3 COUNTER /54STAE 45 STATE 46 OUTPUT REG. /43 /NPUT REG. 5? OUTPUT REc. /aa- //vPc/TREG. /5/

INPUT SW. ACCES$ /52 WRITE IN ORDER WRITE IN ORDER MEM. I MEM. I50 STATE47 SET COU/YTER I54 l" FROM S7247'E 53 STA XE 48 READ OUT ORDER MEM. I50

STATE 49 SH COUNTER /o/ m April 14, 1964 M. C. PAU LL SWITCHING NETWORKCONTROL SYSTEM Filed Nov. 24, 1961 DECREMENT l5 Sheets-Sheet l5 541 5R56. /04 FIG. 24

STAT E 54 STATE 50 I 34 v5 REG. SAVE REG. ourpur sw. 4cc5ss MT 104 NOT-/104: V" OUTPUT INPUT sw. ACCE$S l55 INTERM. sm+4cc5ss //2,/22

READ our NET.

MEM./l0,/20

STATE 55 STATE 51 L READ OUT ORDER MEM. I50

STATE 56 IOUTPUT REG. /55- //VPU7' REG. 1:? INPUT sw.-4ccess uz COUNTER/s4- 4ccess m WRITE IN ORDER MEM- I50 STATE 57 OUTPU T REG. I55 INPUTREG. I5 I OUTPUT SW. ACCES$ I52 COUNTER I54 ACCE$S I53 WRITE/N ORDERMEM./50

/ OUTPUT REG-H3 INPUT REG./5I

OUTPUT SW. ACCESS I52 COUNTER I54 ACCE$$ I53 WRITE IN ORDER MEM. I50

STATE 52 L ourpur REG. 12.: /NPUT REG. m

INPUT SWFACCESS I52 COUNTER /54- ACCE$$ I53 WRITE IN ORDER T v STATE 58SIGNAL ON REQLEAD I7/ ourpur REG. /55 sw. -57: CONTROL l2 w4/r FORACKNOWLEDGMENT 0N LEAD /7/ NO SIGNAL ON RE 0. LEAD I7/ SETS 4a COUNTERSr0 7" TO STATE I United States Patent M 3,129,4ll7 SWITCWG NETWURKCDNTRQL SYTEM Marvin C. Pauli, Denville, NJL, assignor to hell TelephoneLaboratories, incorporated, New York, N.Y., a corporation of New YorkFiled Nov. 24, I961, Ser. No. 154,477 2t? (til. Mil-M7) This inventionrelates to switching systems and more particularly to the control ofmultistage switching networks to unblock paths through such networks bythe physical rearrangement of existing connections.

It is possible that a multistage switching network, by virtue of itsflexibility in the choice of intermediate switches for connectionsthrough the network, may itself be the cause of blocked connections;viz., the inability to complete connections between an input line and anoutput line despite the fact that neither line is already connected.Thus an input line desiring connection through the network has a choiceinitially of a plurality of paths via intermediate stage switches to thedesired output line. However, other lines have access to the sameintermediate stage switches, and in those instances where activeconnections already occupy at least one interstage link in everypossible path between the lines desiring connection, the potentialconnection is blocked.

A variety of nonblocking schemes are available in the art in which thedegree of nonblocking afforded by the network is directly related to thesize and capacity of the switches employed in the network. Thus, with afixed number of input and output lines and a knowledge of the trailicrequirements or expected use of these lines in a given period, thenetwork may be designed to have sulficient capacity to assure that apath will always be available through the network for each desiredconnection.

Such nonblocking proposals are set forth in an article by C. Clos, AStudy of Non-blocking Switching Networks, Bell System Technical Journal,March 1953, pages 406424. Later schemes provided adaptations of the Closnonblocking arrangement which, in general, reduced the switch capacity,while permitting a level of blocking which might be tolerated in thenetwork assigned to perform a particular operation. Such arrangementsare disclosed, for example, in I. C. Gibson et al. Patent 2,945,211,issued July 12, 1960, and in A. Zarouni Patent 3,041,409, issued June26, 1962.

The instant invention marks a radical departure from the basic conceptsset forth in the above-noted nonblocking network arrangements. Thusrather than attempting to provide sufficient capacity to obviateblocking, the system accordance with my invention, upon detecting thepresence of a blocked connection, orders a physical rearrangement ofactive network connections so as to free a path through the network forthe potential connection.

It is therefore a general object of this invention to provide animproved switching system in which blocking conditions encountered inthe switching network may be overcome.

it is another object of this invention to provide a switching systemwhich is economical of switching network components while providing thedegree of operation reliability found in a nonblocking switching system.

it is a further object of this invention to control the operation of ablocked switching network so as to remove the blocking condition in aminimum time without affecting the quality of existing connectionsthrough the network.

It is another object of this invention to reduce the size and spacerequirements for a switching network while rein each stage.

BJZQAW Patented Apr. 14, 1964 taining the quality of service afforded bya completely nonblocking switching network.

These and other objects of the invention are attained in accordance withfeatures thereof by the employment of a network control circuit whichstores an indication of every active connection through the network. Aseach new request for connection of particular input and output switchesthrough the network is received, the network control is firstinterrogated to determine whether or not intermediate stage switchfacilities are currently available to complete the desired connection.If such facilities are available, the network control immediately orderscompletion of the connection, and a record of the newly activatedconnection is stored in the network control. However, if a potentialconnection is found by interrogation of the network control to bephysically blocked, the network control performs a series of logicoperations designed to determine the manner in which the network may bephysically rearranged so as to permit completion of the desired newconnection.

I have found that, in order to unblock the network for completion of apotential connection, no more than nl existing connections through thenetwork must be disturbed where the network contains 11, n n switchesFurthermore, for every 11 greater than 1, there is at least one networkstate in which n1 active connections must be disturbed in order tounblock a blocked connection. The control functions performed by thenetwork control are designed in accordance with these conclusions, suchthat a network rearrangement may be eifected in a minimum time.

Once having determined the active connections which must be taken downand again the particular connections which must be established in orderto free the network for the potential call, the network controlautomatically alerts the network to carry out the orders and to reportback the results. In this fashion the new network pattern is recorded inthe control memory in preparation for the next desired connectionthrough the network.

"in accordance with one aspect of this invention, an auxiliaryintermediate switch is provided such that connections taken down duringthe rearrangement process are established through the auxiliary switchpending completion of the rearrangement in the basic network. In thisfashion the active connections, from the standpoint of the input andoutput lines, remain undisturbed throughout the operation.

It is a feature of this invention that control circuitry be provided topermit completion of an initially blocked connection by rearrangement ofexisting connections through a switching network.

It is another feature of this invention that a network control circuitbe provided which maintains a record of existing network connections anddirects the rearrangement of the network to provide a path for apotential connection which is found, upon interrogation of the networkpattern, to be initially blocked.

It is a further feature of this invention that a control circuit beprovided which will unblock a switching network to a potentialconnection by disestablishing a plurality of existing connections andreestablishing them through switches distinct from those employed in theoriginal connections.

More particularly, it is a feature of this invention that a controlcircuit be provided which will unblock a switch ing network comprisingn, n n switches in each stage so as to serve a potential, initiallyblocked connection by disturbing no more than n-l existing networkconnections.

it is a further feature of this invention that the network controlcircuit respond to those network states in which n-1 connections may bedisturbed in order to unblock 3 the network for a potential connection,there being a finite number of such network states so long as n isgreater than 1.

It is still another feature of this invention that the control circuitrypermit completion of an initially blocked connection through theswitching network consisting of input, intermediate and output switchingstages by altering existing connections so as to disturb no more thantwo intermediate stage switches.

It .is a feature in accordance with one aspect of this invention that anauxiliary intermediate stage switch be provided and that the networkcontrol circuit, concurrently with the disestablishment of connectionspursuant to a rearrangement of the network, establish such connectionsthrough the auxiliary switch so as to maintain the existing connectionsvirtually undisturbed.

A complete understanding of this invention and of the above-cited andother features thereof may be gained from consideration of the followingdetailed description and the accompanying drawing, in which:

FIG. 1 diagrammatically illustrates in simplified form a switchingsystem comprising input, intermediate and output switching stages, linesterminating in both input and output stages, a number of interstagelinks and the common control circuitry associated therewith;

FIGS. 2 and 3 illustrate, mainly in block diagram form, the switchingnetwork control circuit according to the specific illustrativeembodiment of this invention;

FIG. 4A is a diagram of the switching network illustrated in FIG. 1,indicating a particular set of connections established through thenetwork in order to illustrate the specific embodiment of the invention;

FIGS. 4B and 4C are generalized diagrams of the switching networkutilized in the proof of the unblocking proposition;

FIG. 5 is a diagram of the typical content of the output network memoryunit in the network control circuit illustrating the output switchesinvolved in established network connections according to the exampleindicated in FIG. 4A;

FIG. 6 is a diagram of the typical content of the input network memoryunit in the network control circuit illustrating the input switchesinvolved in established network connections according to the exampleindicated in FIG. 4A;

FIG. 7 is a diagram of the order memory unit in the network controlcircuit illustrating a record of the orders to the switching networknecessary to eltect the unblocking of the network to a potentialconnection according to the example indicated in FIG. 4A;

FIG. 8 is a diagram of the output change memory unit in the networkcontrol circuit illustrating particular output switch designations whichit has stored according to the example indicated in FIG. 4A;

FIG. 9 is a diagram of the input change memory unit in the networkcontrol circuit illustrating particular input switch designations whichit has stored according to the example indicated in FIG. 4A;

FIG. 10 is a representation of a logic circuit utilized to compare theoutputs of the network memories illustrated in FIGS. 5 and 6 and toprovide appropriate orders to other circuitry in the network controlcircuit, dependent upon the results of this comparison;

FIG. 11 is a logic circuit utilized to compare the respective outputs ofthe network memories illustrated in FIGS. 5 and 6 with the respectiveoutputs of the corresponding memories illustrated in FIGS. 8 and 9 andto provide appropriate orders to other circuitry in the network control,dependent upon the results of such comparisons;

FIGS. 12A through 1413 are symbols and equivalent circuits fordesignated logic elements utilized in components of the network controlcircuit of FIGS. 2 and 3; and

FIGS. 15 through 24 are diagrams illustrating the timing and progress ofthe network control circuit of FIGS. 2 and 3 in the performance of aparticular network control function and the state of the network controlcircuit during each step toward the realization of the particularfunction.

Before pursuing the detailed operation of the switching network controlcircuit, it may be helpful to consider the theoretical approach to thesolution of the blocking problem through physical rearrangement ofexisting connections in the switching network. It will be assumed forthis purpose that the network consists of n, n n switches in each of aninput, an output and a single intermediate stage. It should he noted,however, that this approach may also be utilized to prove the validityof an unblocking arrangement util izing a plurality of intermediatestages in the switching network or a number of switches in anintermediate stage distinct from the number of switches in the input oroutput stages.

In the three-stage network depicted in FIG. 1, therefore, it is possiblethat a connection between a first lines termination on an input switchand a second lines termination on an output switch cannot be madedespite the fact that neither line is already connected. This couldoccur, for example, if other connections through the net- 'work alreadyoccupy at least one link or connection to an intermediate switch inevery possible path between the input and output lines in question.

The following proof will demonstrate that the unblocking of thethree-stage network to permit such a potential connection will in nocase require the disturbance of more than n-'l existing connectionswhere the size of the switches in each stage is n n and there are nswitches per stage. Thus in the network depicted in FIG. 1 it may beassumed that each input switch I1In has 11 input lines and it links withthe intermediate switches S-lSn. Similarly each output switch 01-0 hasit links with the intermediate stage and n output lines. In order tofacilitate connection, of course, the intermediate switches Sl-Sn alsoare n n switches. in such a network, therefore, a blocked potentialconnection may be unblocked by disturbing n-l existing networkconnections. Furthermore, for every n greater than 1 there is at leastone network state in which n-1 calls must be disturbed in order tounblock the network to serve a blocked potential connection.

Alteration of existing connections in the network will be considered. Inthis regard the input and output switches involved in a particularexisting connection are not altered but only the intermediate switchinvolved in the particular connection. Thus if the network contains aconnection between a certain input line and a certain output line whichis involved in the network alteration to etfect unblocking, theconnection will still exist between the same input and output linesafter the alteration although a difierent intermediate switch will nowbe involved.

The composite network representation in FIG. 4A illustrates some typicalnetwork connections. Thus the input stage of the network is representedby three input switches I1, I2 and I3. Each input switch in turn servesthree input lines. Similarly the output stage of the network comprisesthree output switches O1, O2 and 03, each of which also serves threedistinct lines. The intermediate stage comprises three switches S1, S2and S3, each of which has a link with each of the input and outputswitches thus satisfying the network requirement of n, n n switches ineach stage. Connections through the intermediate stage are representedin the composite network of FIG. 4A by an indication at the crosspointof an input switch row and an output switch column. Thus, for example, aconnection is illustrated as being established between input switch I1,line 2 and output switch 01, line 1 through intermediate switch S1. Asthis connection serves the input switch and line assigned to the firstrow and second level in the composite network representation, thedesignation of the intermediate switch S1 appears in the first row,second level. Also since the connection involves the output switchassigned to the first column and first level of the composite network,the intermediate switch S1 representation will also appear in column 1,level 1. Thus the representation S1 appears at the crosspoint of row 1,level 2 and column 1, level 1 in FIG. 4A.

Each row or column may contain from O to n intermediate switch symbolsinasmuch as each input (output) switch has one link with each of theintermediate switches. Each intermediate switch symbol in FIG. 4A thuscorresponds to a connection from an input switch through the designatedintermediate switch to an output switch. Absence of an entry indicatesthat no connection exists. Although an entry in the matrix alsoindicates which particular input line is connected to which particularoutput line, the specification of the interstage links involved in theconnection is the important property for ultimately achieving anunblocking of the network.

The state of the switching network which will permit unblocking isrestricted to that in which less than all interstage links betweenintermediate and input or output switches are occupied. Of course thesituation in which all intermediate switches are fully utilized presentsa trafiic problem which will not permit a rearrangement of the existingfacilities to provide a path for a potential connection. In the lattersituation a rearrangement could only be effected by the ultimatepermanent removal of an existing connection. Thus in the networkillustrated in FIG. 4A, if all of the intermediate switch designationsS1, S2 and S3 appear in any one row or any one column, the switchingnetwork is full or unequivocally blocked with respect to lines connectedto the input or output switch assigned to that respective row or column,so that an unblocking by physical rearrangement of the networkconnections cannot be realized without destroying an existingconnection.

The type of blocking situation which can be overcome in accordance withthis invention is that in which all intermediate switch designations mayappear in an input switch row and an output switch column together butless than all intermediate switch designations appear in the row orcolumn alone. This corresponds to the case illustrated in FIG. 4A inwhich the input line connected to 11, level 1 desires connection throughthe network to the output line connected to 02, level 2. Row ll containsSi and S3 indicating that input switch ll has previously establishedconnections involving its links with intermediate switches Si and S3.Similarly output switch 02 has a previously established connectioninvolving its links with intermediate switc es S1 and S2. Thus the linesdesiring connection are blocked by prior network connections, but a pathmay be provided by rearranging the network.

Proof For proof of the propositions, reference is made to the compositenet-works illustrated in F168. 43 and 40, which are generalized versionsof the exemplary network illustrated in FIG. 4A. Assume that a potentialconnection between the input switch corresponding to row r FIG. 4B, andthe output switch corresponding to column c is blocked. This impliesthat if unequivocal blocking is ruled out, there is a middle switchdesignated, for example, by the symbol A in column 0 which does notappear in row r Similarly there must be a symbol, say B, in row r whichdoes not appear in column 0 Let A be in (T2, C1) Let B be in (1' C Thusfar we have completely defined:

r the row in which the blocked connection appears,

0 the column in which the blocked connection appears, T the row in whichthe A in column 0 appears, and

c the column in which the B in row r appears.

Now we wish to define other rows and columns:

r the row in which an A appears in column c if there is such a row(otherwise r is undefined),

c the column in which a B appears in row r if there is such a column(otherwise 0 is undefined),

r.; the row in which an A appears in column 0 if 0 is defined, and thereis such a row (otherwise r.,, is undefined),

c.; the column in which a B appears in row r if r is defined and thereis such a column (otherwise c.; is undefined).

In general: for all j l r, is defined to be the row in which A appearsin column c provided c is defined, and provided that A does appear incolumn c If not, r,- is undefined.

0 is defined to be a column in which B appears in row r provided r isdefined, and provided that B does appear in row r If not, c is notdefined.

The above definition has the important property that if r,- and r areboth defined, and j lc, then 1 Also if Cj and c are both defined, and jk, then c c This is justified by the following argument: consider thesequence:

Assume there is a first member equal to a previous member of thesequence.

This is either a row or column.

(1) Assume row r is the first member of the sequence which is bothdefined and the same as a previous defined member, say r #1. First ofall k cannot be 1 since r j 1 is defined to have an A in it, and row rhas no A in it. So r r j l. Now then assume k l, j l, j k. Then an Aappears in (r c and in (r,, c (by our definition of r So unless c :cj-l#kl, there would be two different As in row r :r There cannot be twodifferent As in a single row. Therefore c :c n But this contradicts theassumption that the first member having this property is row r Thatleaves only the possibility of column c being the first such member.

(2) Then assume column 0 is the first member of the sequence which isboth defined and the same as a previous defined member, say column c kThen k l, because c has no B, and 0 j l does by definition of c If k l,j l k j and 0 :0 then for similar reasons to those of the aboveparagraph r n =r Therefore our second assumption, c =c is alsocontradicted, completing the proof.

Having shown that the defined members of sequence (1) are distinct, wewish now to examine this sequence further. For convenience it isrewritten below.

'1 1 '2, i 13,6 r c and the following contain Bs ("1 2); ('a 3); ("r-4r) Now in order to unblock (r c we make the following changes.

Change the original Bs to As in columns c :j=3, 5... iffisodd(orincolumnsj=2,4...iff

'17 This involves changing Bs to As in rows f1 if f is even). r :j=2,4.. f-1 if f is odd (j=1, 3 is even).

Change the original As to Bs in rows r i=2, 4. f-l, if f is'odd (j=3, 5f1 if f is even). This involves changing As to Bs in columns .c j:1, 3,f-2, iffis odd (i=2, 4 f2, iffis even). Note that the total number ofchanges is f1.

We see that if f is odd then after the change (r will still contain a B,but (r c which formerly contained an A now contains a B. Therefore an Amay now be legitimate placed in (r c A similar argument holds if f iseven.

It remains to show that the changes we have prescribed do not lead toany conflicts. For this demonstra'tion assume f is odd. '(A similarargument holds for 1 even.) The only conflicts possible must involve Asand Bs since these are the only symbols changed and resulting from thechange. Furthermore, the only conflicts possible are in rows r r r or 0c c since these are the only rows and columns in which changes weremade. Also at most one A has been added to any row or any column.Similarly at most one B has been added to any row or column. Before thechange there were single As in columns 0,, i=1 to f1, and in rows r i=2to 1. As a result of the change single As were added to column c i:2, fand no others, and to rows r i=2, 4 fl and no others. So it is onlythese columns and rows which couldpossibly contain more than one A. Butthese columns and rows each contain only a single A, because although anA has been added to each, the original A in each has been changed to aB. For according to our prescribed changes, the original As in q: i=1, 3f-2 were changed to Bs. This takes care of all columns to which an A wasadded except column 0;, and column o did not originally contain an A.Also, the original As in rows r i=2, 4 f1 were changed to Bs and thistakes care of all rows to which an A was added.

Again as a result of the change, single Bs were added to rows r i=2, 4,6, fl, and to columns 0 i=1, 3, 5 f2. It is therefore only those columnsand rows which could have more than one B. But the original Bs incolumns Cj, j=3, 5 1 have been changed to As. This takes care of allcolumns to which a B was added except column c and column c originallydid not have a B. Also the original Bs in rows r i=2, 4 f1 were changedAs and this takes care of all rows in which a B was added.

If all members of sequence 1 are defined, then c is the last definedmember, and there cannot be an A in c because such an A would have to bein some row other than row r There are As in all rows other than r butnone of these As are in 0, This follows from the definition of r Fromhere then the argument goes on as the general case in which r was thefirst undefined member of sequence 1.

Thus the maximum number of changes required to unblock the network for apotential connection is nl.

The network, FIG. 40, has n intermediate switches which may berepresented by the symbols A, B, Q Q,, Assume that (1, 1) is blocked bythe following network state:

(i, i); i=1 to 11-1 each contain all the symbols Q (i, i-l-l); i=1 to nleach contain the symbol B. (i-H, i); i=1 to nl each contain the symbolA. There are no other symbols in the matrix. To unblock (1, 1) thesymbols in (.1, 2) and (2, 1) must be made the same because:

(a) After any change there must still be n2 different symbols in (1, 1).

(b) There must still be one symbol in (1, 2) different from all those in(l, 1).

(c) There must still be one symbol in (2, l) diiferent from all those in(1, 1).

(d) If then the symbols in (1, 2) and (2, 1) were different, there wouldbe a total of n symbols in row 1 and column 1, leaving no symbolavailable to unblock (1, 1).

Assume that the symbols in (i+1, i) and (i, i-f-l); i:k1 must be thesame, say X, in order to unblock (1, 1). Now (i, i); i=k, which is inrow k must, after the change, still contain n2 different symbols, say YY Y n The symbolX in (i-l-l, i); i=kl which is also in row k must bedifferent from Y Y Y Therefore the symbol in (i, i+1); i=k which is alsoin row k must be different than X, Y Y

Y There is only one symbol that can be different from all nl differentsymbols X, Y; Y say Z. So Z must appear in (i, i+1), i=k+1. Similarly asstated previously (i, i), i=k, which is in column k, must still have then2 different symbols Y Y Y Also in column k the symbol X is in position(i, i+1), i=k1. Therefore it follows that the symbol in (i+1, i), i=k,which is also in column k must be different than X, Y Y,, and must be Z.

Hence the induction is complete proving that if (i, i) i=1 to n 1,. eachcontain n2 difierent symbols (this must be true because of the giveninitial network state) and (l, 1) is to be unblocked then for each i=1to n-1, the pair (i+1, i) and (i, i-l-l) must contain the same symbol.Since initially (i+1, i) contained a different symbol from (i, i+l) fori=1 to 11-1, at least n--l changes are necessary to put the network in astate both equivalent to its initial state, and in which (1, 1) isunblocked.

Switching Network For the particular illustrative embodiment of theinvention described hereinafter, a three-stage network is chosen, asdepicted in FIG. 1. The switching arrangement is of course not ofimmediate concern inasmuch as the invention resides primarily in thecontrol of a multistage network rather than in the network itself. Thusthe switching components of the network and their interconnection maytake any one of a variety of forms, a crossbar network arrangementfamiliar to the art being illustrated in FIG. 1. For the sake of a clearunderstanding of the network control to follow, however, the arrangementand operation of the depicted crossbar switching network will bedescribed in brief.

Each of the input (I), intermediate (S) and output (0) switching stagesincludes a plurality of crossbar switches I1ln, S1Sn, 01-011. Eachswitch in turn is represented by a plurality of vertical pathsintersected by a plurality of horizontal paths. Each vertical andhorizontal path is intended to symbolize a plurality of conductors andeach intersection or crosspoint of a vertical path with a horizontalpath symbolizes a plurality of circuit establishing devices; e.g., relaycontacts, whereby electrically conductive paths may be selectively setup between the corresponding conductors represented by any vertical pathand any horizontal path.

Each vertical path in each input and output switch is connected to adistinct communication line. The horizontal paths in each input switchare linked in a slip-multiple pattern to the horizontal paths in theintermediate switches in such a manner that each input switch horizontalis connected to a corresponding horizontal in a distinct intermediateswitch. Likewise, each intermediate switch vertical path is linked to acorresponding horizontal path in a distinct output switch.

With the flexibility provided by the intermediate stage in the switchingnetwork, a communication line terminating in a particular input switchhas access to a communication line terminated in a particular outputswitch via any one of the intermediate stage switches S1Sn having idlelinks between the particular input and output switches. Thus, forexample, consider that the communication line represented by the lineequipment lLO desires a connection to the line represented by equipmentOLn, the former being terminated in ingot switch ll and the latter beingterminated in output switch Oil. The network control circuit proceeds todetermine the availability of an intermediate switch Sl3n havingavailable its links with both ll and 01, considering of course that bothinput and ou Jlll'. lines are presently idle.

if switch S1 satisfies these requirements, the COHIIGCilOIl will becompleted from 110 through vertical path and horizontal path 0 of inputswitch 11, through horizontal path G and vertical path 0 of intermediateswitch S1, and through horizontal path 0 and vertical path n of outputswitch Gl to the desired output line terminal OLn.

Of course since a number of lines lLO-lLn and OLO-OLH have access to theswitches ll and O1 simultaneously, the occasion may arise when line lLOcannot be connected to the desired output line ()Ln due to the fact thatone or both links between the respective intermediate switches Sl-Sn andeach of the input switch lit and the output switch Cl are serving otherconnections. In normal operation this network condition would bereflected to the input line by a busy signal and the connection lost.According to the instant invention, however, existing network conneconemay be rearranged by breaking one or more of the links and rerouting thedisturbed connections through different intermediate switches.

Thus, for example, a call established between input switch ll and outputswitch 01 through intermediate switch S2 may be contributing to tireblocking condition of a potential connection involving another lineconnected to input switch ll. The blocked condition may be removed bymerely transferring the prior connection from intermediate switch S2 toanother intermediate switch, for example, S1, so that the priorconnection is undisturbed at its terminal switches ll and 01 but nowincludes intermediate switch 51 rather than S2. In this fashion the linkbetween ii and SE and the link between S2 and Oi are now free toaccommodate the desired potential connection of ill and G1.

The manner in which the cross connections are established in theswitching network is well known in the art; i.e., utilizing the identityof the input and output lines to be connected to activate the particularhorizontal and vertical paths in each switch involved in a connection.Thus each vertical path in the input and output switches is controlledby a ccrrespon flip-flop circuit lVO IViz and OVO-OVn. Similarly, theindividual input and output switches are controlled by flip-flopcircuits lSQ 18:1 and OSC-OS:1. in this fashion the particular verticalpath and the input or output switch corresponding to a particular linemay be identified. Also the combination of fliplops SSO-SSn andhorizontal gate circuits HG]. and HGZ further identifies a particularintermediate switch to be associated with input and an output switch tocomplete a connection through the network.

Network control ilbll registers indications of the switch and verticallocations of the calling and called lines and determines the desired itermediate switch for completing the connection through the network. Thedevices indicated by rectangles and ill, designated input line identityand output line identity, respectively, do not constitute an essentialpart of the present disclosure but are ancillary thereto and are shownmerely to indicate a suitable source of calling and called lineidentifying indicia to be supplied to the network control Thusrectangles llll and Ill represent any suitable device well known in thecommunications art for providing calling and called line identifyingindicia in a form suitable for registration. The network control lullreceives this information, compares it with the state of the switchingnetwork as stored therein, and provides similar identification of input,intermediate and output switches in the form of orders to ll) thecontrol circuit 12 to either establish or disestablish networkconnections.

A more detailed analysis oi the manner in which input and output linedesignations received from external equipment may be utilized toestablish or disestablish a network connection is set forth in the citedZarouni patent application referred to hereinafter as Zarouni. Theintermediate switch connections in Zarouni are effected upon completionof a scanning routine which is utilized therein to determine availableintermediate switches. in the instant case such information is supplieddirectly to the control circuit by the network control so that thescanning operation is circumvented. The manner [for such circumventionand completion of the intermediate switch connections is apparent fromconsideuation of the cited Zarouni application.

The instant circuit also requires indications as to when a desiredoperation has been completed and a connection established ordisestablished. This information is also available in Zarouni, asindicated by the presence of ground on the sleeve leads of connectedinput or output lines, which ground may be readily extended through theswitches to the network control 1%. Removal of such ground of courseindicates that a disconnect has been effected.

Thus all of the information essential to the operation of the networkcontro in accordance with this invention, in order to operate theswitching network, may be derived from the circuitry disclosed inZarouni, and the instant disclosure hereinafter will be confined to thecontent and manner of operation of the network control Hill itself.

Network Control Referring now to H68. 2 and 3 of the dnawing, aschematic diagram in block form of one specific illustrative embodimentof the network control llltl in accordance with this invention is shown.As there depicted, the circuit contains an output network memory 11d andan input network memory 120, each of which comprises a matrix of memoryelements familiar to the art; e.g., magnetic cores. The elements areaccessible on a coincident current basis for purposes of writing newinformation in the memory or reading information nondestructively fromthe memory. Such a memory is disclosed, for example, in Digital ComputerComponents and Circuits by R. i. Richards, D. Van Nostr and Company,inc, 1957, page 388, PEG. 8-15, and page 393, referred to hereinafter asRichards.

Conside ing memory 116, the coordinate access circuits ill and 112comprising suitable devices for translating the identity of an inputswitch and an intermediate switch, respectively, activate apredetermined discrete element of the memory lllll in a distinct timeinterval. Dependent upon which output register 113 or input register 114is activated in the same time interval, information is respectively readfrom or written in the discrete memory element. Memory ll acts insimilar fashion to store input switch designations in response to outputand intermediate switch access designations.

Each discrete memory element has the capacity to store a plurality ofbinary digits forming a word representing a particular line identity.Thus the memory is said to be word organized, as known in the art, anddisclosed, for example, in Richards, page 33, FIG. 85. Input, output andaccess circuitry suitable for the memory circuits in this embodiment ofthe invention are also described in Richards, page 57, FIG. 2-12.

The memories llllll and 12d serve as an instantaneous record of thenetwork condition. As each new connection is completed through thenetwork, the particular output switch involved in the connection isrecorded in the memory lllll at a point which is accessible from theparticular input and intermediate switch indications also involved inthe connection. For example, in order to ascertain the identity of anoutput switch involved in a

1. IN A SYSTEM COMPRISING SWITCHING APPARATUS ARRANGED IN A MULTISTAGENETWORK, CONTROL MEANS COMPRISING MEANS FOR OPERATING THE SWITCHINGAPPARATUS TO ESTABLISH A PLURALITY OF LINE-TO-LINE CONNECTIONS THROUGHTHE NETWORK, MEANS FOR DETECTING THE PRESENCE OF A BLOCKING CONDITION INTHE NETWORK WITH RESPECT TO A POTENTIAL CONNECTION AND MEANS FOROPERATING THE SWITCHING APPARATUS WHILE SAID ESTABLISHED CONNECTIONS AREACTIVE TO EFFECT A REARRANGEMENT OF CERTAIN OF SAID PLURALITY OFESTABLISHED CONNECTIONS TO REMOVE THE BLOCKING CONDITION.